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2.5_IR_spectroscopy_post - Spectroscopy Infrared Electric...

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2. IR Spectroscopy 1 Spectroscopy Infrared Electric Field Magnetic Field ± w a v e l n g th Electric wave Magnetic wave Direction of motion of the light beam Light is an electromagnetic phenomenon. A beam of light consists of two mutually perpendicular oscillating fields: an oscillating electric field and an oscillating magnetic field λ

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2. IR Spectroscopy 2 Equations νλ = c λ ν c = c = frequency ( ν ) number of cycles per second (units: cycles s -1 = Hertz, Hz) wavelength ( λ ) distance spanned by one cycle (units: meters, m) Wavenumber number of cycles in 1 cm or inverse of the wavelength in cm (units: cm -1 ) speed of light ( c ) 2.99892458 x 10 8 m s -1 in a vacuum Planck’s constant ( h ) = 6.626076 x 10 -34 Joules s Larger the frequency smaller the wavelength Larger the frequency larger the wavenumber 1 = wavenumber The fundamental mechanism responsible for this radiation is the acceleration of a charged particle. Whenever a charged particle undergoes acceleration, it must radiate energy at the speed of light as packets of energy called photons. The energy of a photon is inversely proportional to the wavelength hc E = or Smaller the wavelength or larger the frequency the larger the energy of the photon lower energy, smaller frequency, larger wavelength, smaller wavenumber higher energy, greater frequency, smaller wavelength, larger wavenumber h E =
2. IR Spectroscopy 3 Spectroscopy What is it? Spectroscopy detects transitions between the quantized energy levels of an atom or molecule, where the energies are due to some internal motion of the atom or molecule. In atoms, these motions are due to the changes in the electronic configuration . In molecules these internal motions arise from changes in either rotations (microwave spectroscopy) vibrations (infrared spectroscopy), electronic excitations (uv-visible and photoelectron spectroscopy) or nuclear spin orientation (nuclear magnetic resonance). How do we do it? Transitions are observable by measuring discrete frequencies of electromagnetic radiation that are absorbed or emitted by atoms or molecules. Why do we do it? By characterizing the energy levels of a molecule, we can describe the geometry of that molecule, including the lengths and strengths of its bonds, the identity and positions of its constituent atoms, the energies of its molecular orbitals, and the overall framework and shape of the molecule. (Chem 129 notes by Dr. B. Power. 1997)

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2. IR Spectroscopy 4 Regions of the electromagnetic spectrum and the types of molecular motions 100 cm (cm-1) 108 1 cm 10 -2 10 -1 10 -3 10 -3 104 106 11 0 0 100 m ± 1 m ± 10 nm wavenumber 10 m 100 pm wavelength 3 x 10 6 3 x 10 8 3 x 10 10 3 x 10 12 3 x 10 12 3 x 10 14 3 x 10 18 10 10 3 10 5 10 7 10 9 frequency (Hz) energy (J/mol) low energy high energy STD broad cast TV microwaves IR Visible UV x-rays gamma rays gamma rays Change in spin Nuclear Magnetic resonance Electron spin resonance Change of orientation Microwaves Change of configuration Infrared Change of Electronic distribution visible and ultra--violet X-ray Change in nuclear configuration
2. IR Spectroscopy 5

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2.5_IR_spectroscopy_post - Spectroscopy Infrared Electric...

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